Wireless radio communication is subject to the adverse effects of signal fading, in which the signal level at the receiver temporarily loses strength for a variety of reasons, such as from variable multipath reflections causing signal cancellation, time varying transmission loss due to atmospheric conditions, and mobile receiver movement introducing obstructions into the signal path, and the like. Signal fading causes poor reception, inconvenience, or in extreme cases, a loss of call connection.
It is known to use various forms of signal diversity to reduce fading. For example, as indicated in U.S. Pat. No. 5,280,472, signal diversity mitigates the deleterious effects of fading. There are three major types of diversity: time diversity, frequency diversity and space diversity.
Time diversity is obtained by the use of repetition, interleaving or error correction coding, which is a form of repetition. Error detection techniques in combination with automatic retransmission, provide a form of time diversity.
In frequency diversity, signal energy is spread over a wide bandwidth to combat fading. Frequency modulation (FM) is a form of frequency diversity. Another form of frequency diversity is code division multiple access (CDMA) also know as spread spectrum. Due to its inherent nature as a wideband signal, CDMA is less susceptible to fading as compared to a narrow band modulation signal. Since fading generally occurs in only a portion of the radio spectrum at any one given time, a spread spectrum signal is inherently resistant to the adverse effects of fading.
Space diversity is provided by transmitting or receiving the same signal on more than one geographically separated antennas. Space diversity provides alternate signal paths to guard against any one path being subject to fading at any one time. Space diversity also creates some time diversity since the receiver receives the same signal separated by small propagation delays. The difference in propagation delay requires that the receiver be able to discriminate between the arriving signals. One solution is to use multiple receivers, one for each arriving signal. For instance, it is known from U.S. Pat. No. 5,280,472 to deliberately introduce relatively small delays compared to an information symbol, into a space diversity multiple antenna CDMA system in order to create artificial multipath time diversity signals greater than one chip delay up to a few chips. CDMA systems are capable of discriminating between identical plural signals arriving at the receiver with different propagation delays greater than one chip delay. Such receivers are known as Rake receivers. However, prior art systems require multiple CDMA receivers, one CDMA receiver for each separate received CDMA signal. It is desirable to provide a system for receiving time diversity CDMA signals which does not require multiple CDMA receivers.
Measuring or determining the location of mobile units is well known. In some systems, fixed antennas measure the mobile location. In other systems, the mobile unit determines its location from multiple received signals. If the system is two way, the communication link permits both the mobile subscriber and the fixed system to exchange location data. Various known systems use satellites or multiple antennas to provide information on the location of a mobile subscriber. For example, multiple directional receiving antennas can be used to triangulate the position of a mobile transmitter. In such systems, the stationary receivers determine the mobile subscriber location; in other systems, the mobile subscriber determines its location from the received signals. For example, the Global Position System (GPS) is a multiple satellite system providing signals which permit a mobile subscriber station to determine its position in latitude and longitude. However, both satellite systems and the GPS receivers for receiving satellite signals tend to be expensive.
The combination of a GPS receiver and a cellular telephone is shown in U.S. Pat. No. 5,223,844. Such combination provides useful services, as for example a security alarm service to deter car theft, in which tripping the alarm also alerts the security service to the location of the car. Generally, it is desirable to provide a system which combines telephone or data service with location measurement at a reasonable cost.
It is desirable to provide a system of time diversity signals using time division multiple access (TDMA) in various combinations with CDMA and space diversity antennas, to provide a variety of systems which resist fading, reduce receiver cost, and provide location measurement for mobile subscribers.